Subject: 29.0.50; Generic functions and advertised-calling-convention

From: "Basil L. Contovounesios" <[email-masked]>
Date: Sun, 16 Oct 2022 13:24:30 +0300

Further to https://bugs.gnu.org/58531#25, generic functions do not
currently support advertised-calling-convention very well (or vice
versa).

For example, starting with:

  (cl-defgeneric my-foo (x &optional _y)
    "Frobnicate X."
    (declare (advertised-calling-convention (x) "29.1"))
    x)

Any code that calls my-foo with two arguments correctly gives rise to a
warning during byte-compilation.

C-h f also shows the expected arglist, but not for methods:

  my-foo is a Lisp closure.
  (my-foo X)
  Frobnicate X.
  This is a generic function.
  Implementations:
  (my-foo X &optional _Y)
  Undocumented

More importantly, if we now do:

  (cl-defmethod my-foo ((x symbol) &optional _y)
    "Frobnicate X the symbol."
    (declare (advertised-calling-convention (x) "29.1"))
    x)

Then my-foo's symbol-function is overwritten and its entry in
advertised-signature-table is no longer found, so byte-compilation no
longer warns about incorrect usage, and C-h f regresses to displaying:

  my-foo is a byte-compiled Lisp function.
  (my-foo X &optional Y)
  Frobnicate X.
  This is a generic function.
  Implementations:
  (my-foo (X symbol) &optional _Y)
  Frobnicate X the symbol.
  (my-foo X &optional _Y)
  Undocumented

Note that, unlike with cl-defgeneric, the declare form in cl-defmethod
does not expand to a call to set-advertised-calling-convention.  If
set-advertised-calling-convention is called after the cl-defmethod, then
the advertised-calling-convention is preserved (or rather reinstated),
but only until the next cl-defmethod is defined (which could happen in
third-party code).

I guess either advertised-signature-table should be extended to allow
for the nature of generic functions, or cl-defmethod should be taught to
preserve such function properties (or both).

I wonder if cl-defgeneric should be the single source of this function
property, or whether any cl-defmethod should be able to overload it.
Thoughts?

Thanks,

-- 
Basil

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